1. Field of the Invention
The present invention relates to apparatus and methods for grinding materials into fine particles. More particularly, it relates to those apparatus methods in which a basket mill is used.
2. Background Art
A basket mill is a machine used to reduce the particle size of slurries. Typical uses are for the manufacture of paint or ink where pigment particles must be reduced to microscopically small sizes.
Such mills can accommodate a wide range of volume depending on the volume and viscosity of the slurry to be processed. Typical volumes range from 0.5 liter for a table-top mill to as much as 40 liters for a full manufacturing mill.
Referring to FIG. 1, such machines typically have a process vessel 10 mounted to a dolly-like structure shown generally as 12 having four casters 14, so that process vessel 10 may be moved along a shop or other floor when grinding apparatus in accordance with the prior art, or according to the invention, as described below with respect to FIG. 2, is withdrawn upward from process vessel 10. Similarly the process vessel can be supplied without casters and therefore maintained in a fixed position. In general, a basket 16 is loaded with a large number of small beads (not shown). A mixture of the material being ground and the small beads are stirred by agitator discs or pins 18, which extend radially from a rotating agitator shaft 20. The beads typically can be from 0.5 mm to 6 mm in diameter. The beads can be made of various materials including glass or steel or ceramic. These beads are collectively known as “grinding media”, and are well know in the art.
Thus, the agitator shaft 20, which has radially arranged discs or pins 18, is immersed into the grinding media and stirs the bead mass while a slurry 22 of the material being ground is introduced. It is the movement of the beads impacting against the particles of the material being ground which acts to reduce the particle size. This process is also known as “wet grinding”. The basket 16, together with its contained grinding media, is immersed into a process vessel 10, which contains the slurry 22.
The slurry 22 is made to flow into the top of basket 16 through its generally open top 24 at 25. The beads are retained in the basket while allowing the slurry to pass through because the sidewall 26 and bottom wall 28 of the basket 16 are actually made of a screen material. In effect the container resembles a large basket, thus the name “basket mill”.
Generally, there have been two designs of basket mills. In a first design, as illustrated in FIG. 1, the agitator shaft 20, driven to rotate by a suitable drive assembly 30 including a motor and an appropriate speed control for the motor has, mounted at the bottom of the drive shaft, an impeller 32 which serves, when it rotates with the agitator shaft 20, to draw slurry into the top of the basket 16, as illustrated by arrow 33. In another design (not shown), the bottom of the basket is closed, and one or more separate impellers are mounted alongside the basket.
In either of these prior art designs, the flow of slurry 22 into the basket 16 is dependent on the action of the impeller. The first type of machine, described above with respect to FIG. 1, is generally preferred due to the presence of the center-mounted impeller 32, but has numerous maintenance problems. Since the same shaft is use for both the agitator discs or pins 18 and the impeller 32, a stabilizing bushing 34 is required where the agitator shaft 20 passes through the bottom of basket 16. The bushing 34 is immersed in the grinding media mass. The attack of the grinding beads on the bushing 34 leads to frequent bushing replacement, because the bushing 34 is repeatedly worn by the action of the grinding media inside the basket. Further, the impeller 32 and agitator discs or pins 18, mounted on the same shaft 20, by necessity, must spin at the same speed.
With the separate propellers of the second design, the propeller is separate, so there is no need for the bottom mounted bushing of the first design which is under constant attack by the action of the grinding media However, the offset nature of the propeller does not provide the better flow, and control over the flow of material as in the first design.
Accordingly, there is a need for a better apparatus, and a method for its operation, which permits precise control of flow, without significant maintenance difficulties.